The advantages of fluidized bed thermal treatment, specifically, high fuel efficiency, uniform heating or cooling, safety, and low cost are well known.
Typical fluidized bed devices include the following principal parts: a reaction vessel having a fluidized bed portion, a particulate solid heat transfer media, i.e. fluidized bed media, disposed within the bed portion of the reaction vessel and a gas distributor plate. The gas distributor plate causes a separately supplied fluidizing gas, e.g. air or nitrogen to pass upwardly through the fluidized bed media suspending the bed media particles therein, creating an expanded mass. The expanded mass exhibits thermal properties usually associated with low viscosity liquids, particularly, high heat transfer rates and high temperature uniformities. The temperature of the expanded fluidized bed media may easily be controlled by heating or cooling the reaction vessel and/or the fluidizing gas. A workpiece may be rapidly heated or cooled by submersion in expanded bed media maintained at a predetermined temperature.
Applications of fluidized beds in heat treating processes involving preheating of raw materials prior to other steps, e.g. forming operations, curing, or thermosetting, etc., have been complicated or altogether defeated by problems resulting from conventional refractory fluidized bed media, clinging to the heated workpieces and being carried into subsequent process steps. For example, the advantages of fluidized bed preheating are often very desirable in metal forging, rolling or extruding processes etc., however, known bed media are abrasive refractory particles such as aluminum oxides, and silica sands which, if they cling to the heated workpiece, can cause damage to the equipment performing subsequent process operations and/or cause defects in the finished product. Attempts to eliminate this problem calling for special treatment of the workpiece or raw material before or after the preheating steps have met with varying degrees of success. The major disadvantage of these approachs is that they invariably involve additional equipment and process time leading to significantly greater production costs.